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The Physics Behind a Great Wheel:
Frog Legs has 15 years of experience designing and manufacturing just caster technology. Attention to even the smallest detail has resulted in the Lightest, Strongest, Most Functional Wheel in the world developed specifically for wheelchairs.
These four charictaristics will profoundly effect the way your char will ride.
1) Tread Width
2) Tread Arch Ratio
3) Tread Material
4) Wheel Diameter
Tread Width:
A wider tread will perform a “snowshoe” effect by better distributing the downward pressure over a larger surface so the wheel stays on top rather than penetrating to the bottom. This is also known as “ floatation”.
Tests conducted at Iowa State University have determined that a 1.5" wheel will decrease the force of impact by 21% and roll over 20% more obstacles than a narrow 1" wheel. A wider, arced tread will do a better job of holding debris in place allowing the wheel to roll over it rather than catching and turning the wheel sideways.
Floatation is the displacement of pressure exerted by the wheel’s contact patch (the tread in contact with the ground). A standard 1” wide caster wheel has approx. a ¼” contact patch whereas a 1.5” wheel has upto a 1” wide contact patch providing 3 to 4 times the area to displace the force. This means that a narrow wheel will produce 75% MORE downward force.
Tread Arch Ratio:
Front wheels on a wheelchair are passive. This means that they are not critical to either steering or accelerating the chair. Because of this, wheelchair caster wheels should not have tread and should be designed without flat sidewalls.
The goal for a wheelchair front caster is to be as close to spherical as possible( aka a ball). In scientific terms the arch from side to side should be the same as the arch of the circumference. Imagine a golf ball in heavy carpet - no matter how much weight is applied to the ball, it stills wants to roll because the contact patch is completely round.
The arched tread allows the wheel to pivot and roll much easier.
Tread Material / Urethane Formula:
To understand urethane you need to understand two terms: 1) durometer and 2) rebound.
Durometer:
Durometer is used to describe the softness of the urethane. The higher the number associated, the harder the wheel will be. Softer wheels will provide more cushion and roll over more everyday debris. Conversely, hard wheels will slide easier and are considered faster on super smooth surfaces.
According to Jeff Vyain (considered an expert in longboarding), “The big question is middle ground…..Too soft, and your wheel may not have enough rebound to be really fast. Too hard, and you’ll collide into inconsistencies in the road rather than roll right over them”.
Rebound / Hysteresis:
Urethane has “shape memory” meaning that it can compress and then return to its original shape after being distorted. You can test the rebound of a urethane wheel by dropping it on the floor and seeing how high it bounces.
Rebound improves the efficiency of a wheel because it will minimize the rolling resistance. Your body weight will compress the urethane wheel some at the leading edge. As the wheel rolls, the back edge will rebound returning the energy used to compress the wheel into forward momentum. The net result is rebound reduces the frictional drag of the wheel. A hard wheel will simply crash into debris in its’ path and slow down.
The amount/speed of rebound is important. The more rebound, the faster the wheel will roll. The speed of rebound comes into play when the urethane deforms to catch and roll over an obstacle. It needs to rebound at a rate exceeding the angular velocity to reform its’ shape and continue without slowing down. This is why ureathane foam and recycled ureathane are unacceptable - their memory lag is too slow to efficiently return energy.
Wheel Diameter:
 In most sporting arenas conventional wisdom is larger = faster & smaller = quicker acceleration. This is because smaller wheels have less mass and therefore less rotational inertia to overcome to begin turning. Larger wheels achieve higher road speed for the same rotational speed because there is less frictional drag from the bearings.
However, this does not apply for wheelchair applications because the front wheels are passive (ie the large rear wheels perform all the acceleration). For wheelchair applications the critical consideration is “the larger the wheel, the larger the obstical it will roll over”. The secondary aspect to consider is turning radius. You do not want a wheel so large it extends out beyond your handrail when turned sideways. This can make getting on/off ramps and thru narrow spaces more difficult and too large of a wheel sweep can interfere with the rider's feet or the wheelchair's footplate.
A wheel (without a FrogLeg-style shock absorber) cannot roll over an obstacle unless the height of the obstacle is smaller than the radius of the wheel. Obstructions seem bigger to a small wheel because they hit it at a higher point on its arch and force it to climb steeply to get over. Large wheels hit the same obstructions at a lower point and therefore have more vertical forece / leverage to roll up and over. A large wheel also spans holes and cracks in surfaces better.
In short, an additional 1 inch diameter can make a huge differenc e in your ability to climb over debris rather than getting stuck.
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